1931 Leica I (C) originally had
non standardized lenses that were custom fitted because the distance
from the lens flange to the film plane was not a standard distance.
Later examples had the distance standardized and an 'O' was engraved on
the lens mount to signify that the lens mount was standardized. This
model is the first Leica with interchangeable lenses. This
standardized example has an early interchangeable Elmar with the
infinity lock at the 11 o'clock position instead of the later 7 o'clock